Subsea Pipeline Free Span Detail Fatigue Analysis

[atedionyahoo]

Ref.: DNV-RP-F105 (Feb 2006)

•Question 1
Sec.: 4.2.1 & 4.2.2:
Could somebody literally explain the meaning of “integration over the long term distribution for current velocity represented by a Weibull distribution or histogram”. (If possible to provide a simple example, please)

•Question 2
Sec. 4.2.2 (marginal fatigue life capacity – In Line VIV):
I think the Eq 4.2.2 shall be modified since the frequency of VIV pure IL is different with CF induced IL frequency. I understand the fatigue life should be minimum between pure IL and CF induced IL.


Could somebody kindly advise on either of the above questions.

 

[BigInch]

Current velocities are usually assumed to have a Weibul probability distribution. You must consider the accumulated fatigue from all current velocities that will occur during the design lifetime of the pipeline, caused by the time that each velocity is expected to be present during the pipe design lifetime. You must calculate the frequency generated by each current velocity multiplied by the time each velocity is in effect and take their sums to arrive at the total number of cycles that the pipeline (or segment) will experience during its lifetime.

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[nietzche]

Regarding Question 2
Sec 4.2.3 explains this.

The idea is to estimate the damage separately for IL and cross flow induced IL with different frequencies as defined and 4.2.3 and then compare and take the conservative among the two for Inline damage.

Eq in 4.2.2 should be read together with the 4.2.3, but i see your point, the concept could be made more clear by moving the frequency term fv inside the brackets and by using different notations for both (say, fvIL and fvCFIL).

Hope this clarifies your query.

 

[atedionyahoo]

Thanks for replies.

Re Question 1)
I know that wave and current effects in fatigue should be considered. My matter is mathematical as I do not know how to calculate dF(Uc) in Sec 4.2.1 or 4.2.2. (F(Uc) is cumulative weibull distribution of current) (d is differential)

However, I think mathematically it can be calculated as follows:

I get cumulative weibull distribution of current which is function of Uc, i.e. F(Uc). Then instead of dF(Uc) in Sec 4.2.1 or 4.2.2, I use its equal ( dF(Uc)/dUc ) * dUc , in this way I can calculate the integral formula in Sec 4.2.1 or 4.2.2 over the whole Uc,

Any advise?

Re Question 2)
I see eye to eye with you.